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Wave motion is an important method of transferring energy from one place to another without involving the actual transfer of matter. When a pebble is dropped into a still pool, some of the kinetic energy of the pebble is used to generate the ripples which spread out in all directions over the surface of the pool. When the ripples pass by a floating object, such as a bit of cork, the cork bobs up and down, having acquired its kinetic energy of vibration from the ripple system. The ripples thus serve to deliver some of the energy of the pebble to the distant cork. Although the ripple system is seen to move, there is no net flow of the water, for the cork simply bobs up and down and does not move in the direction of the ripples. Since the cork merely provides visible evidence of the behavior of the water, we may infer that the motion of the water is one of vertical oscillation, while the motion of the wave is horizontal; that is, the oscillations of the water are transverse to the direction of propagation of the wave. A wave motion in which the vibrations of the medium are perpendicular to the direction of propagation is called a transverse wave. The waves set up in a taut string when one end of the string is vibrated in simple harmonic motion are transverse waves.